On Tuesday, the Socialist majority in the French parliament is expected to approve François Hollande's flagship social reform, the gay marriage and adoption bill, making France the 14th country in the world to legalise same-sex marriage. It is the most significant social reform since France banned the death penalty in 1981.

But, in recent days opposition to the bill has spilled over into violence, with riot police teargassing people protesting against same-sex marriage and arresting more than 100 people after nightly demonstrations near parliament. Last week, skinheads attacked a gay bar in Lille, injuring several people after announcing they had "come to beat up gays". A gay bar in Bordeaux was attacked by masked, armed men the same night. Politicians in favour of gay marriage have received death threats and there was a near fist-fight in parliament between MPs over the bill.

The anti-gay marriage movement has been building for six months, after its first major demonstration was the largest gathering of conservative and rightwing protesters in France for 30 years. But policing was stepped up for Sunday's demonstration after the interior minister warned of the appearance on the edge of the movement of small rightwing extremist groups which have caused tensions and differences among demonstrators in recent days. Manuel Valls said the groups, thought to number at most a few hundred of people, adhered to the "Vichy ideology" of France's Nazi-collaborationist regime during the second world war.

A comedian Virginie Tellene, better known by her stage name Frigide Barjot, is leading the anti-gay marriage street marches. She condemned homophobic attacks and said radical groups were not welcome at the demonstrations. At the start of the rally, organisers told police of a demonstrator carrying six teargas canisters who was detained.

The group SOS homophobie, which has monitored homophobia in France for 20 years, said the sharp rise in homophobic incidents since the start of the national debate on gay marriage was "unprecedented".

The debate in both upper and lower chambers of parliament have been of unprecedented verbal violence, which campaigners said had allowed homophobic insults to proliferate on the streets. This week the justice minister called the parliamentary debate "a spaghetti western" and an MP from the main rightwing opposition UMP party accused the government of "killing children" by allowing gay adoption. Earlier a senator said legalising gay marriage was akin to allowing people to marry "animals" or "objects".

At the demonstration, Catholics, students, lawyers, families, teachers and shopkeepers marched behind a handful of rightwing politicians, including one MP elected on a ticket with the far-right Front National's Marine Le Pen.

Thibault Genin, 32, who had been in the French military and now worked as a consultant, said: "It's an old technique of the French left to call you a Nazi if you don't agree with them. It's rubbish. We will fight to the end against this law.

"I'm absolutely not a homophobe, but children and family rights are at stake. I led my troops into combat for France, so now to be called a Nazi by my own government, or teargassed by police for staying at a demonstration past an appointed time is hard to take."

At the parallel anti-homophobia demonstration, Laurent Delaire, a nursing auxiliary, and Gilles Le Berre, a railways ticket inspector, described how on the night of a major anti-gay marriage rally in March, neighbours knocked on their door shouting homophobic insults. The couple, who plan to marry, informed the police. "Initially we felt tense walking down the street, but we're not scared, you can't give in. It's absolutely unacceptable the type of language that MPs and senators have been using," Le Berre said.

Laure Pora, from the Aids awareness group Act Up, said: "There's an increase in homophobia today in France. It has always existed but it wasn't always expressed so freely. Now the political debate and demonstrations have led to a kind of 'legitimation' of homophobic comments. We're on the street to say we have every right to be here, the street doesn't belong to people who would prefer us dead."

The latest poll by BVA showed 58% of French people supported the right to gay marriage but 53% opposed the right to adoption for gay couples.